In recent years an increase in communication traffic has promoted introduction of WDM (Wavelength Division Multiplex) optical transmission apparatus. The number of ring networks or mesh networks using such optical transmission apparatus has increased.
If small-number channel transmission using a small number of channels is performed in such an optical network, the fact that OSNR (Optical Signal-to-Noise Ratio) in the small number of channels falls by the influence of PHB (Polarization Hole-Burning) and that the transmission characteristics deteriorate is known.
Accordingly, an optical transmission apparatus which reduces the influence of PHB and which controls an OSNR degradation is proposed (see, for example, Japanese Laid-open Patent Publication No. 2009-290593). This optical transmission apparatus reduces the influence of PHB in the following way. Of channels (no-signal channels) via which a signal is not transmitted, an attenuation amount in a VOA (Variable Optical Attenuator) in a channel adjacent to a channel (signal light channel) via which a signal is transmitted is made small and an ASE (Amplified Spontaneous Emission) light generated by amplification on the upstream side is transmitted along the channel.
In addition to a currently used system, an optical network includes a spare system as a redundant line in order to avoid a communication failure such as the slipping off of a fiber cable or the breaking of a line. A redundant line is, for example, an OUPSR (Optical Unidirectional Path Switched Ring). An optical transmission apparatus monitors the power of a signal light. When the power of a signal light becomes equal to or lower than a determined threshold, the optical transmission apparatus determines that a communication failure, such as the breaking of a line, has occurred, and switches a line from a currently used system to a spare system.
With conventional optical transmission apparatus, however, the following problem arises. For example, if switching from multichannel transmission to small-number channel transmission is performed due to a communication failure such as the breaking of a line, an ASE light may be transmitted along a channel which changes from a signal light channel to a no-signal channel. In this case, a downstream optical transmission apparatus recognizes an ASE light transmitted along the channel as a signal light. As a result, the downstream optical transmission apparatus cannot detect the communication failure, such as the breaking of a line, and perform switching to a redundant line.